This invention relates to multiple coil incandescent filaments, and more particularly, to a single strand filament wire, coreless, triple-coil filament for an incandescent lamp.
The light output of incandescent lamps is proportional to the light-emitting surface area of the light source. An incandescent lamp having a triple-coil filament, that is, a coiled-coiled-coil filament is advantageous as a light source in that it allows for an approximate three fold increase in the light-emitting surface area relative to a single filament and when arranged into an incandescent lamp provides an improved efficacy of the incandescent lamp.
Triple coil filaments are known in the art of incandescent lamp and are taught, for example, in United Kingdom (UK) Patent Specification No. 475,006 of Nov. 16, 1936 of E. I. Lonn and our U.S. Pat. No. 4,316,116, issued Feb. 16, 1982.
UK Patent Specification No. 475,006 discloses a triple-coil filament having various diameters and coiling cores for the first, second and third coils of the triple-coil filament. The UK Patent Specification No. 475,006 disclosing desired properties which may conceivably yield a triple-coil filament having an outside diameter in the range of (26.1 to 50.6) d, where d is the diameter of the filament wire.
A triple-coil filament having an outer diameter in the range of 26.1d to 50.6d presents problems relative to maintaining the geometry of the filament during lamp operation. It has been empirically determined that for such sized filaments having an outer diameter between 26.1d to 50.6d stress relieving or recrystallization processes need to be performed to the filament before arranging the filament within the incandescent lamp so that the geometry of the filament is maintained during the operation of the incandescent lamp.
Maintaining the geometry of the filament during incandescent lamp operation is needed so as to prevent the filament from sagging when the temperature of the filament is elevated to its operating temperature. If sagging is not prevented, the separated windings of the triple-coil filament may come into contact with each other which causes the filament to short-circuit itself and experience a burn-out. The undesirable filament burn-out is most pronounced for the inner coil spacing associated with the first wound coil of the triple-coil filament. It is considered desirable to provide a triple-coil filament not having the disadvantages related to the triple-coil filament of UK Patent Specification No. 475,006.
Our U.S. Pat. No. 4,316,116 teaches the incandescent art a triple-coil incandescent filament for the incandescent lamp having certain desired mandrel ratios so as to provide the desired triple-coil filament. Our U.S. Pat. No. 4,316,116 teaches a triple-coil filament having a maximum outside diameter which is less than 27d, where d is the diameter of the filament wire. Still further our U.S. patent teaches a triple-coil filament having a minimum outside diameter equal to 15d.
It has been determined in the practice of the teaching of U.S. Pat. No. 4,316,116 and by empirical data that the 27d outside diameter triple-coil filament is not particularly suitable for incandescent lamps. Triple-coil filaments having an outside diameter of 27d have been found to be of a flimsy structure which when arranged within the incandescent lamp without a recrystallization or stress relieving treatment experience sagging, which, in turn, as previously discussed with regard to the triple-coil filament of UK Patent Specification No. 475,006, causes the separated winding of the triple-coil filament to contact each other, which, in turn, causes the filament to experience a burn-out. The undesirable filament burn-out is most pronounced for the inner coil spacing associated with the first wound coil of the triple-coil filament.
In a manner similar to that described for the triple-coil filament of our U.S. Pat. No. 4,316,116 having an outer diameter of 27d, it has been determined by the practice of our invention of U.S. Pat. No. 4,316,116 and also by empirical data that the triple-coil filament of U.S. Pat. No. 4,316,116 having an outer diameter of 15d experiences problems related to maintaining its desired geometry within and during the operation of the incandescent lamp. More particularly, for the triple-coil filament having an outside diameter of 15d the first coiling pitch ratio has to be increased to a quantity which causes the lamp performance, that is, the efficacy of the lamp, to be reduced, and thus becomes less desirable.
It is considered desirable to provide the incandescent lamp art with a triple-coil filament having preselected dimensions that eliminates the need of recrystallization of the triple-coil filament prior to arranging the triple-coil filament within the incandescent lamp.
Accordingly, it is an object of the present invention to provide a triple-coil filament lamp allowing for a compact design and having design parameters further allowing for the integrity of the coil geometry of the triple-coil filament to be placed into an incandescent lamp without the need of prior recrystallization of the tungsten filament before lamp assembly.
Further objects and features and a more complete understanding of the present invention which may admit to a number of possible variations will be seen from the following description of our invention in conjunction with the attached drawings, representative of preferred embodiments of our invention.